Field of the Invention
The present invention relates to a toner useful for an electrophotographic system, an electrostatic recording system, an electrostatic printing system, and a toner jet system.
Description of the Related Art
Following wide spread of full-color copiers of an electrophotographic system in recent years, a demand has grown for further improvement of image quality and reduction in energy consumption. In the electrophotographic system, a printed matter is generally obtained through a development step, transfer step, and fixing step.
From the standpoint of saving energy, it is desirable to develop a toner that can be melted more rapidly and at a lower temperature than the conventional toners, thereby enabling rapid fixing at a low energy in the fixing process which consumes a large amount of power for heating.
To meet such a requirement, it is necessary to soften the toner, but from the standpoint of heat-resistant storability and durability, such an objective cannot be attained simply by softening the binder resin of the toner particles which is contained in the toner.
Further, a problem associated with the diversification of electrophotographic printing in recent years is that when continuous printing is performed on paper types of various sizes, for example, when printing on A4 paper is performed immediately after printing on small-size paper such as postcards and envelopes, the temperature of the fixing member portion which is not in contact with the paper rises thereby causing a hot offset.
Accordingly, Japanese Patent Application Publications No. 2004-061875 and 2011-123352 have suggested toners with low-temperature fixability improved as a result of including a composite resin constituted by an amorphous resin and a crystalline polyester resin having a sharp melt property.
Where the content of the crystalline polyester resin is increased, the low-temperature fixability is further improved, but the resin strength and durability degrade. As a result, the anti-hot offset property and storability are lost due to thermal stresses and mechanical stresses. Further, since an external additive on the surface is freed or embedded due to the decrease in resin strength, the charging performance and flowability are degraded, and a problem of melt adhesion at parts such as a photosensitive member easily occurs during printing.
Another problem is that since charges leak through the crystalline polyester resin, the amount of charges on the toner is decreased and the image is distorted.
Meanwhile, methods of increasing compatibility of the crystalline polyester resin and amorphous resin have been suggested for improving the low-temperature fixability while reducing the content of the crystalline polyester resin.
For example,
Japanese Patent Application Publication No. 2012-118499 suggests reducing the molecular weight of the crystalline polyester resin;
Japanese Patent Application Publication No. 2004-286842 suggests including a lubricating agent; and
Japanese Patent Application Publication No. 2013-242523 suggests adjusting the SP value of the resin.
In an example of Japanese Patent Application Publication No. 2012-118499, a crystalline polyester resin is described which has a molecular weight as low as 6100, but this is insufficient for realizing a high image quality which is presently required, while maintaining the low-temperature fixability of the toner.
Further, Japanese Patent Application Publication No. 2014-006339 suggests a toner in which the degree of crystallization is increased and both the low-temperature fixability and the charge stability are ensured by adding a nucleating agent to a crystalline polyester resin. In an example of Japanese Patent Application Publication No. 2014-006339, since the molecular weight of the crystalline polyester resin is as high as 17,000 or higher, compatibility with the amorphous polyester resin serving as a binder resin is insufficient, and the low-temperature fixability cannot be attained at a level which is presently required to further increase the operation speed and reduce energy consumption.
As mentioned hereinabove, in the field of toners having toner particles including a crystalline polyester resins, there is still room for investigation aimed at ensuring excellent low-temperature fixability and also charging performance, storability, and anti-hot offset property in combination with increase in speed and improvement in image quality.